It is known to use a hydrophilic filter to filter liquid as it flows in a passageway for intravenous administration or the like. There is always a problem of gas or air intrained in the liquid reaching the filter and air locking it impeding further flow. One of the ways in which this problem has been attacked is to insert in a filter chamber holding the hydrophilic filter a hydrophobic membrane to vent air or gas that may collect in the filter chamber, the gas-free liquid then passing through the hydrophilic filter. There is, nevertheless, a possibility that the hydrophobic membrane, being fragile, may rupture and cause contamination of the liquid, or if the patient's venous pressure at the administration site is low enough, that air may be driven into the system through the hydrophobic vent. This air may then be accidentally infused into the patient's circulatory system.
Various arrangements have been proposed for the application of a secondary or "piggyback" solution higher than the primary solution, to be preferentially administered to the patient. Such arrangements usually involve a branch from the passageway through which liquid is provided from the principal or main solution. The branch leads to a piggyback solution. It has also been proposed to employ various valves or the like to assure the preferential administration of the secondary solution until exhaustion whereupon the flow of the primary solution takes up or begins again. The following patents may be considered as exemplary: U.S. Pat. No. 3,886,937 of June 3, 1975 to Bobo, et al; U.S. Pat. No. 3,982,534 of Sept. 28, 1976 to Buckman; and U.S. Pat. No. 3,993,066 of Nov. 23, 1976 to Virag. Note also the U.S. Pat. No. 3,854,907 of Dec. 17, 1974 to Rising. A prior art patent which exemplifies the venting of air or gas through a hydrophobic filter from a liquid being filtered for a patient is the above-identified Rising patent. Note also U.S. Pat. No. 4,013,072 to Jess. Prior patents which exemplify mechanical valving for administration of so-called "piggyback" solutions are U.S. Pat. No. 3,886,937 issued June 3, 1975 to Bobo, et al, and U.S. Pat. No. 3,982,534 issued Sept. 28, 1976 to Buckman, the latter also having means for removing air bubbles from the flow of liquid, and U.S. Pat. No. 3,993,066 issued Nov. 23, 1976 to Virag.